1. Field of the Invention
The present invention relates to a glass antenna device for vehicles which is suitable for a receiving set on a vehicle, and which is used for receiving signals in radio broadcast, TV broadcast, telephone communication and satellite communication or the like.
2. Discussion of Background
There has been known a glass antenna device (a first conventional glass antenna device) which, as shown in FIG. 28, includes a glass sheet 31 for an automobile rear window, an electric heating type defogger 33 arranged on the glass sheet and having heater strips 32 and bus bars for supplying power to the heater strips 32, an antenna conductor 136 which is capacitively coupled to the defogger 33 in proximity thereto so as to make transmission of a direct current therebetween and to prevent transmission of a high frequency current therebetween, and a choke coil 139 connected between the bus bars and a d.c. power source 10 for the defogger 33.
In the first conventional glass antenna device, the choke coil 139 is inserted between the bus bars and the d.c. power source 10 to increase impedance of the choke coil 139 in a high frequency band region such as a broadcast frequency region, thereby allowing a direct current to flow from the d.c. power source 10 to the defogger 33 and preventing currents in the broadcast frequency band region from flowing.
By this arrangements the choke coil 139 can isolate the defogger 33 from a vehicle body ground in terms of high frequencies, and prevent high frequency received current induced in the defogger 33 from flowing to the vehicle body ground, thereby transmitting the high frequency received current to a receiving set 20 in their entireties.
As a conventional glass antenna device (not shown) other than the one shown in FIG. 28, there has been a glass antenna device (a second conventional glass antenna device) wherein an antenna conductor for an AM broadcast frequency band (hereinbelow, referred to as the AM band) and an antenna conductor for an FM broadcast frequency band (hereinbelow, referred to as the FM band) are separately arranged in a glass sheet for an automobile window in order to improve receiving sensitivity. The second conventional glass antenna devices create a problem in that the antenna conductors occupy large areas to deteriorate a visual field.
In the first conventional glass antenna device, a decrease in inductance due to magnetic saturation has to be avoided in order to interrupt a high frequency current because a high direct current for the defogger 33 flows in the choke coil 139 at tens of amp. As a result, adoption of a special winding structure is normally required.
Since the inductance of the choke coil 139 is normally as large as 0.6-1 mH, and since the value of a d.c. resistance is required to decrease to minimize power loss, the winding of the choke coil 139 has to use a coil having a large diameter.
As explained, when the defogger 33 is used as an antenna to ensure good receiving sensitivity, the provision of the choke coil which has a large structure and which is large-sized and heavy, and measures to avoid receiving sensitivity in the FM band are required, creating problems in that the entire structure of the glass antenna device is complicated and productivity is low.
As to the second conventional glass antenna device, since, in receiving AM broadcast, the antenna conductor for the AM band is used without utilizing the antenna conductor for the FM band, receiving efficiency is low. It has been desired to develop a glass antenna device which can use both antenna conductors to improve receiving sensitivity and receive radio waves efficiently.